Hydraulic brake systems for automobiles and other wheeled and tracked vehicles are known in the art. A four-wheel hydraulic disc service brake system 10 commonly used in passenger cars is illustrated in simplified schematic form in FIG. 1. Brake system 10 includes brake pedal 12 that serves as the interface between the vehicle's operator and the rest of brake system 10. By controlling brake pedal 12, the vehicle's operator can use brake system 10 to slow or stop the vehicle.
Brake pedal 12 is mechanically coupled to master cylinder 14, as would be understood by one skilled in the art. Master cylinder 14 is hydraulically coupled via brake lines 16 to brake calipers 18, each of which is operably associated with a corresponding one of the vehicle's four wheels. Each brake caliper 18 also is operably associated with a corresponding brake rotor 20 through a pair of brake pads (not shown), as would be understood by one skilled in the art. Each brake rotor 20 is attached to a corresponding wheel hub (not shown) so that it can rotate with the corresponding wheel hub and wheel (not shown).
When brake pedal 12 is depressed, master cylinder 14 acts to displace and pressurize brake fluid in brake lines 16 and brake calipers 18, causing the brake pads (not shown) to be pressed against the corresponding rotor 20, as would be understood by one skilled in the art. Friction between the brake pads and rotors 20 tends to slow and/or stop the rotors, if rotating, and to maintain the rotors at rest, if not rotating. When brake pedal 12 is released, master cylinder 14 acts to allow the brake fluid in brake lines 16 and calipers 18 to return toward its original state, in turn allowing the brake pads to return toward their original states, as would be understood by one skilled in the art.
Operation of system 10 requires only the application to and release of force from brake pedal 12. No other external power, for example, electrical or engine power, is necessary to operate system 10. Although system 10 may further include some form of boost system, for example, a vacuum boost system, to lessen the physical effort required for operation of system 10, as would be understood by one skilled in the art, such a boost system is not required. Further, even if such a boost system were provided, the boost system would not need to be operable in order for system 10 to otherwise be operable. As such, system 10 is generally operable at all times except when the system is out of service for maintenance or in the event of significant component failure.
As an adjunct to service brake system 10, passenger cars typically also include some form of parking brake system. Although, such parking brake systems may share certain components with the vehicle's service brake system, they typically are actuated by purely mechanical means (for example, using a cable), not hydraulically, using a parking brake lever or a parking brake pedal that is separate from and independent of brake pedal 12. Operation of such parking brake systems typically requires only the application of force to the parking brake lever or pedal and associated release mechanism. No external power is necessary.
Certain hydraulic service brake systems used in trucks, buses, and the like are more complicated. One such hydraulic service brake system 100 is illustrated in simplified schematic form in FIG. 2. Similar to the typical passenger car service brake system 10 described above and illustrated in FIG. 1, system 100 includes brake pedal 112 coupled to master cylinder 114 in a conventional manner and brake calipers 118 operably associated with corresponding brake rotors 120 in a conventional manner. System 100, however, also includes a number of additional components not found in system 10, and system 100 operates in a significantly different manner than does system 10.
In addition to the components set forth above, system 100 includes manifold 122, hydraulic pump 124, electric pump motor 126, and gas-filled hydraulic accumulator 128. Manifold 122 is hydraulically coupled to master cylinder 114, brake calipers 118, hydraulic pump 124, and accumulator 128.
In use, pump motor 126 drives hydraulic pump 124, which pumps brake fluid from a reservoir (not shown) into accumulator 128, thereby pressurizing accumulator 128 with brake fluid. A control unit (not shown) controls the operation of pump motor 126 in response to signals received from pressure sensors (not shown) associated with accumulator 128 to maintain the brake fluid pressure in accumulator 128 within a predetermined range.
When brake pedal 112 is depressed, master cylinder 114 acts to displace and pressurize the brake fluid in brake lines 116A coupling master cylinder 114 and manifold 122, thereby actuating relay valves (not shown) in manifold 122, as would be understood by one skilled in the art. So actuated, the relay valve hydraulically couples accumulator 128 and brake calipers 118 through brake lines 116B, 116C, thereby displacing and pressurizing the brake fluid in brake lines 116B, 116C and brake calipers 118, in turn causing the brake pads (not shown) to be pressed against the corresponding rotors 120, as would be understood by one skilled in the art. When brake pedal 112 is released, master cylinder 114 acts to allow the brake fluid in brake lines 116A to return toward its original state, in turn allowing the relay valves to return toward their original states, thereby hydraulically uncoupling accumulator 128 and brake line 116B from brake lines 116C and brake calipers 118. With brake lines 116C and brake calipers 118 thus uncoupled from accumulator 128 and brake line 116B, a relief valve (not shown) bleeds brake fluid from brake lines 116C and calipers 118 back to the reservoir (not shown), thereby de-pressurizing brake lines 116C and calipers 118 and allowing calipers 118 and the brake pads to return toward their original states, as would be understood by one skilled in the art.
An optional parking brake system (not shown) can be provided as an adjunct to service brake system 100. Such a parking brake system typically would include a parking brake caliper and rotor and means to actuate and release the parking brake caliper. Typically, such a parking brake caliper would use a spring to press the brake pads against the rotor and would use hydraulic pressure provided by accumulator 128 and pump 124 and controlled by a valve (not shown) associated with manifold 122 to release the brake pads from the rotor, as would be understood by one skilled in the art.
Unlike brake system 10, brake system 100 requires external power for normal operation because the hydraulic pressure that actuates calipers 118 to press the brake pads against the rotors ultimately is generated by pump 124, which is driven by electric pump motor 126. (Embodiments including the foregoing, optional, parking brake system also require hydraulic pressure from the same source to release the parking brake pads from the parking brake rotor as discussed above.) Pump motor 126 typically would be operable with the vehicle's ignition switched “on” and operating power, for example, battery power, available and inoperable with the vehicle's ignition switched “off” and/or operating power unavailable.
Although accumulator 128 stores hydraulic energy for use in normal operation of system 100, the amount of hydraulic energy stored by accumulator 128 is limited and typically would be sufficient to effect only a few brake applications (or releases of the parking brake). Further, over an extended period of non-use, the energy stored in accumulator 128 could bleed down to a level insufficient for normal operation of brake system 100. With pump motor 126 and, therefore, pump 124, inoperable and accumulator 128 drained of sufficient, stored hydraulic energy, brake system 100 is incapable of actuating calipers 118 (or releasing the parking brake caliper, where provided).